How To Make Your Own Bonsai Wire: Annealing Copper

If you’ve been reading this site, you may have noticed a certain DIY attitude that I like to follow when it comes to bonsai. Why? Because it’s fun, that’s why. Being involved in every aspect of whatever interest you choose to pursue will help you to not only understand things better but also to become much more skilled at your craft and troubleshooting when things go wrong, which they always eventually do. Bonsai is unique in that you are constantly learning, regardless of your experience. Horticulture, sculpture, gardening, and now metallurgy!

So to get to the point, why would you want to use copper wire when aluminum is so cheap and easy to acquire? All I can say is just give it a try. There’s a reason why, despite the expense of copper and cheapness of aluminum, copper is still used by so many experienced bonsai artists. Its main advantage is that it’s strong. Really strong. Plus it looks beautiful on trees, unlike aluminum. And because of its strength it doesn’t need nearly as much to achieve the same results as a piece of aluminum wire. There are drawbacks though, such as the price. And it takes more skill to work with, as it hardens quite significantly once in place, making it much more difficult to remove. But when copper wire is heated hot enough it becomes very soft, just as soft as aluminum until it’s worked.

Today we’ll be solving a few of these problems in one swoop. Price, availability, and softness. We are going to anneal some scrap electrical copper wire to use it for bonsai. Annealing is simply heating plain copper wire until it reaches in excess of 760 degrees fahrenheit or 405 degrees celsius. Many metals such as copper have a cubic crystal structure. This structure is what lets you bend and twist copper so easily, making it seem soft and ductile. As you deform the copper, defects in this crystal structure are introduced. These defects are what make the copper begin to stiffen and strengthen, hardening it. This property of copper is why it’s excellent for bonsai. It’s very soft as you are wrapping it around your branches, making it easy to apply. Once it’s twisted and bent, it hardens and is very strong, keeping your branches just where you want them.

scrap wire still insulated

I have some scrap electrical wire from various past projects that I’ll be using today. I’ve also bought a small roll of 18 gauge 3 strand wire because I needed some more 1 mm wire for smaller branches. Since wire is not commonly sold in millimeter sizes, but gauges instead, and millimeter is a little easier for most people to understand when it comes to wire thickness, see the following chart for conversions.

use this guide to convert from AWG to millimeter

Also, if you are unsure what size your wire is and it is insulated, there should be some numbers printed along the insulation. With this yellow wire you can see it is 12 gauge, which according to the chart is approximately 2 mm.

notice the small number printed on the insulation to find the gauge of your wire

What I like about using electrical wire is that usually you will have multiple strands within, each one corresponding to ground and positive, and sometimes neutral as well. This means a 20 foot section of wire with 3 strands will actually yield 60 total feet of wire. This comes out much cheaper than not only buying specialty wire for bonsai, but also cheaper than buying copper wire by the foot at your local hardware store.

Most insulated electrical wire will have a bit of nylon fiber within. If you pull on this fiber, it will separate the outside insulation sheath from the wire itself, leaving you with your separate wires.

unsheathed copper electrical wire

After unsheathing the wire, coil it around something in order to straighten it and make it easier to anneal. Here I’m using a cardboard shipping tube that I’ve cut the end of. Coil it around the tube and secure the coil by taking an end of the wire and wrapping it around itself so the coil does not become unwound.

coil the wire around a cylinder and secure it

uninsulated wire that has been coiled for annealing

For larger gauges of wire, it is much better to cut into long straight lengths before annealing. Remember that when winding the wire around your branches it begins to stiffen and strengthen, holding the branch in place. With thicker wire, just the act of uncoiling the wire before applying to your tree can make it too stiff to work with before you even get it on the branch. Cutting it into straight lengths beforehand will allow you to coil it around your tree while it is still soft, with no need to straighten from the initial coil.

a 6 gauge wire, roughly 4 mm in diameter, will be cut into straight lengths

Now that your wire is cut into straight lengths or rolled into coils depending on its thickness, it’s time to prepare a heat source. I like to use a small outdoor fire pit. I get it started with some oak wood, then lay charcoal on top of that.

my small pit ready to fire

After you’ve lit your fire, wait for the flames to burn down and leave you with a bed of red hot coals. This will be what you begin to anneal your copper on. If you are unable to strip off all of the insulation, you can lay the coils directly on to the coals and the plastic will burn off of the wire. It’s important that your wire begins to glow red hot. This will let you know that they are hot enough to remove from the fire and have been successfully annealed. It may be easier for you to do this at night in order to see the glowing of the copper.

laying your copper wire onto the hot coals

Use something to fan the coals as you wait for the wire to get hot enough. I do my straight wire last, and lay the coals on top of the wire as I fan the fire to increase the heat.

several straight lengths of large gauge copper wire are covered with coals

After your wire is hot enough, use a pair of pliers or tongs to remove it and quench it quickly in a bucket of water. This makes it easier to set it aside without catching anything on fire or burning yourself.

You will notice that your newly annealed wire is no longer a pretty copper color. It’s covered with soot from the flames and remnants of plastic insulation.

the newly annealed wire is tarnished and unattractive

This is easily remedied with a simple solution of vinegar and salt. Use one cup of vinegar with two tablespoons of salt in a pot. Fill the rest with water and bring to a boil with your wire inside.

use salt and vinegar to clean your newly annealed copper

After the copper wire becomes clean, remove from the pot and dry thoroughly. What you’re left with is nice clean annealed copper wire.

your newly annealed copper bonsai wire is now clean and ready to use

You can test it by bending the end of some of the wire and noticing how soft it seems. Once you bend it enough it will begin to stiffen considerably. You are ready to start wiring you trees with the exact same kind of copper wire that’s sold from bonsai specialty shops but that you have created for a fraction of the cost. And you also have something to brag about at your next bonsai club meeting!